Myotonic dystrophy (DM) is caused by nucleotide expansions of CUG and CCUG in the non-coding regions of the dystrophia myotonia protein kinase gene (DMPK) and the Zn finger 9 gene (ZNF9) respectively. Patients with the CUG expansions have type I myotonic dystrophy (DM1) and patients with the CCUG expansions have type II myotonic dystrophy (DM2). Patients with DM1 and DM2 display the same symptoms, suggesting both CUG and CCUG expansions cause DM through the same mechanism. The hypothesis for how these non-coding expansions cause DM is through an RNA gain-of-function mechanism;the expanded CUG and CCUG repeat RNAs sequester the MBNL RNA binding protein and also indirectly increase the protein levels of another RNA binding protein (CUG-BP), which disrupts the normal cellular function of MBNL and CUG-BP. MBNL and CUG-BP are pre-mRNA splicing factors that appear to function antagonistically. Changing their "active" concentration results in the mis-regulation of alternative splicing of multiple transcripts with a final outcome of DM for people with CUG and CCUG expansions. The focus of this proposal is to determine the role of MBNL in the disease state (DM) as well as in normal cells. To accomplish these goals we are using a combination of bioinformatics, biochemical, biophysical and structural methods. Aim 1. Identify cellular targets of MBNL and determine the mechanisms through which MBNL regulates pre- mRNA splicing. Aim 2. Biochemical and structural characterization of CUG and CCUG repeats alone and in complex with MBNL. Our goal is to understand the molecular mechanisms that cause DM. This understanding will help lead to the development of therapies to help the many people (1 in 8000) that have this most common form of adult onset muscular dystrophy.